This application claims the priority of German Patent Application No. 198 02 631.5, filed Jan. 24, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a process for purifying exhaust gases of an internal-combustion engine. In addition, the invention relates to a system for purifying exhaust gases of an internal-combustion engine.
In order to reduce the pollutant emissions of an internal-combustion engine (for example, a diesel or Otto engine), such an engine can be equipped with an emission control system through which the exhaust gases flow. For purifying the internal-combustion engine exhaust gases, NO.sub.x -adsorber systems are particularly suitable. Under certain conditions, such exhaust gas purification elements, which are also called NO.sub.x adsorber catalysts, store the nitrogen oxides (NO.sub.x) of internal-combustion engines when they are operated in a "lean" manner. Such a lean operation exists if the combustion air ratio lambda (.lambda.) is larger than 1 (i.e., when there is an overstoichiometric combustion, during which large amounts of oxygen are present in the exhaust gas). For the regeneration of such NO.sub.x adsorber systems which, because of their storage capability are also called storage catalysts, an exhaust gas is required that has a reducing effect and a reducing agent content that is as high as possible. This results in the NO.sub.x stored in the NO.sub.x adsorber catalyst being released and converted to nitrogen N.sub.2. An internal-combustion engine produces exhaust gas that has a reducing effect when a "rich" combustion is present (that is, an understoichiometric combustion with .lambda.&lt;1), during which no residual oxygen or only little residual oxygen exists in the exhaust gas.
The internal-combustion engines equipped with such an NO.sub.x storage catalyst must therefore have an engine control system that permits a change between a lean operation and a rich operation of the internal-combustion engine.
During the lean operation, the exhaust gases of the internal-combustion engine contain sulfur oxide compounds (SO.sub.x), preferably sulfur dioxide (SO.sub.2), which react with the storage material of the NO.sub.x storage catalyst and in the process form sulfates. Such sulfate formation leads to a reduction of the NO.sub.x storage capacity of the NO.sub.x storage catalyst. This is also called "sulfur poisoning" of the NO.sub.x storage catalyst.
So that an exhaust gas purification system with an NO.sub.x storage catalyst can function properly over an extended time period, the sulfur content in the exhaust gas must be reduced. The essential sulfur sources are the fuel and the engine oil. Thus, fuels and engine oils with a lower sulfur content increase the useful life of the NO.sub.x storage catalyst.
The sulfate formation in the NO.sub.x storage catalyst can also be avoided if an SO.sub.x storage catalyst (also called an SO.sub.x trap) is arranged in the exhaust gas line in front of the NO.sub.x storage catalyst. When the exhaust gases flow through the SO.sub.x storage catalyst, a large portion of the sulfur compounds emitted by the engine are adsorbed and stored therein. In this manner, the durability of the NO.sub.x storage catalyst is considerably improved.
However, the SO.sub.x storage capacity of such an SO.sub.x trap or SO.sub.x storage catalyst is limited so that regeneration or desulfurization of the SO.sub.x storage catalyst must be carried out for a continuous operation. Such a desulfurization can be achieved by means of an exhaust gas which contains a reducing agent (such as CO, H.sub.2, HC) and has a relatively high temperature. Under these conditions, the previously stored sulfur quantities are mainly desorbed as SO.sub.2 and H.sub.2 S and released, in which case the SO.sub.x storage capacity of the SO.sub.x storage catalyst is restored.
The present invention has the object of further developing a process of the initially mentioned type such that the exhaust gas composition and exhaust temperature required for the desulfurization of the SO.sub.x storage catalyst can be provided by technically simple measures and devices.
This object is achieved by means of a process according to the present invention.
The present invention is based on the general idea of varying the exhaust gas composition by means of the engine control such that it has a reducing atmosphere which, for the SO.sub.x storage catalyst, causes a release of the SO.sub.x compounds. The high exhaust gas temperature also required for this purpose is reached in a simple manner by means of feeding secondary air into the exhaust gas line, behind the engine and in front of the SO.sub.x storage catalyst. Here, the exhaust gas enriched by reducing agents contains a high chemical energy which, while oxygen is fed, can be converted to thermal energy by means of corresponding chemical reactions. The oxygen required for this purpose is made available with the secondary air. In the SO.sub.x storage catalyst, a portion of the reducing agents carried along in the exhaust gas catalytically combusts with the oxygen of the secondary air, during which the thermal energy is released and is preferably transmitted to the surface material of the SO.sub.x storage catalyst. The high temperature in the SO.sub.x storage catalyst required for the sulfate decomposition can therefore be generated by this chemical reaction in the SO.sub.x storage catalyst itself and therefore requires no additional energy source.
An atmosphere containing reducing agent is provided in the exhaust gas in a simple manner. As the result of the engine control, a change is made from the lean operation to a rich operation of the internal-combustion engine.
In order to be able to obtain an optimal desulfurization, preferably a temperature or more than 550.degree. C. is set in the SO.sub.x storage catalyst.
In order to be able to achieve such a high temperature in the SO.sub.x storage catalyst and in order to achieve a composition of the exhaust gases which is optimal for the desulfurization of the SO.sub.x storage catalyst, the combustion air ratio of the exhaust gases mixed with the secondary air is selected from a range of .lambda.=0.75 to .lambda.=0.99.
The setting of these preferred values for the combustion air ratio of the exhaust gases mixed with secondary air and for the temperature existing in the SO.sub.x storage catalyst corresponding to a preferred embodiment of the present invention is achieved in that, during the desulfurization, the engine control influences or varies the quantity of the fed secondary air and/or the combustion air ratio of the exhaust gases coming from the engine. This permits in a simple manner an automatic control or control of the parameters which are characteristic of desulfurization.
In the case of an exhaust gas purification system, in which the SO.sub.x storage catalyst is arranged in the exhaust gas line in front of the NO.sub.x storage catalyst, the sulfur compounds released during the desulfurization of the SO.sub.x storage catalyst arrive in the NO.sub.x storage catalyst and can form compounds there with the NO.sub.x storage material and form sulfates. This has the result that the NO.sub.x storage capacity of the NO.sub.x storage catalyst is reduced.
The problem therefore occurs of carrying out the desulfurization of the SO.sub.x storage catalyst such that in the process the storage capacity of the NO.sub.x storage catalyst is not impaired. This is achieved in that a bypass is provided in the exhaust gas line which bypasses the NO.sub.x storage catalyst and which is activated during the desulfurization by the engine control. By means of this bypass, the exhaust gases loaded with the sulfur compounds are directed away from the NO.sub.x storage catalyst during the desulfurization so that no sulfate formation can occur in the NO.sub.x storage catalyst.
In another, particularly advantageous embodiment of the process according to the present invention, the adsorption of sulfur compounds in the NO.sub.x storage catalyst during the desulfurization of the SO.sub.x storage catalyst can be prevented in that, after the change-over from the lean operation to the rich operation of the internal-combustion engine, a regeneration of the NO.sub.x storage catalyst is carried out. The engine control monitors a parameter which correlates to the degree of regeneration of the NO.sub.x storage catalyst, and only when a predetermined threshold value for this parameter is reached, secondary air is fed into the exhaust gas line. By means of this preceding regeneration phase, with the aid of the reducing agents emitted by the engine during the rich operation, the oxygen quantities and nitrates stored in the SO.sub.x storage catalyst and in the NO.sub.x storage catalyst are converted. As the result, the two catalysts (SO.sub.x and NO.sub.x storage catalyst) are changed to a reduced condition, in which, except for the sulfates in the SO.sub.x storage catalyst, approximately no more oxygen-containing atoms or molecules exist in the catalysts. After such a regeneration, particularly of the NO.sub.x storage catalyst, the actual desulfurization of the SO.sub.x storage catalyst can then take place in that secondary air is fed. In the case of an immediately following desulfurization, the sulfur compounds adsorbed and stored during the lean operation are desorbed and released from the SO.sub.x storage catalyst. The released sulfur compounds can flow through the reduced NO.sub.x storage catalyst without the possibility that an adsorption or storage of the sulfur compounds can take place. Sulfur poisoning or sulfurization of the NO.sub.x storage catalyst can therefore be prevented during the desulfurization of the SO.sub.x storage catalyst connected in front, specifically exclusively by the selection of a particularly skillful course of the control and automatic control operations. An exhaust purification system operating according to this process has few movable components and is therefore robust, not very susceptible to disturbances and reasonable in price.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.